Electromagnetic measuring device



Oct. 20, 1953 P. F. ELARDE ELECTROMAGNETIC MEASURING DEVICE 2 Sheets-Sheet 1 Filed July 11, 1945 //vvE/\/T0/E PE EL HEDE Patented Oct. 20, 1953 UNITED STATES FATENT OFFICE ELECTROMAGNETIC MEASURING DEVICE Application July 11, 1945, Serial No. 604,456

2 Claims.

This invention relates to an electromagnetic measuring device and more particularly to a magnetic flux measuring apparatus.

The measurement of the flux density of magnets is very difiicult where close tolerances are required. Where critically narrow limits are prescribed, the conventional method of measuring flux, using a single search coil and fluxmeter, presents difficulties in reading the deflection of the fluxmeter with the required precision because the tolerances may approximate the observational error. For example, if a certain magnet gives a total deflection of 100 mm. on the fiuxmeter and the required tolerance is i1 mm. in terms of the fiuxmeter, the observational error may be approximately the same, and, therefore, mask the true reading. If the total deflection were only that due to the tolerance, then the observational error would not interfere with the measurement because it is only a small percentage of the total deflection.

An object of this invention is to provide an efficient and effective electromagnetic measuring device.

In accordance with one embodiment of the invention, the flux value of a magnet under test is measured by rotating, through 180, 9, pair of coils connected in series-opposing relation and in series with a fluxmeter, both coils simultaneously cutting two different magnetic fields. One of the coils cuts the field of a magnet to produce a known change in the number of flux linkages, while the second coil cuts the field of a second magnet whose flux value is to be determined. The coils being in series-opposing, the fluxmeter will register the difference between the known change in the number of flux linkages produced by the first coil and magnet and the change in the number of flux linkages produced by the second coil cutting the field of the magnet under test.

A complete understanding of the invention may be had by reference to the following description taken in conjunction with the accompanying drawings, wherein Fig. 1 is a plan view of the apparatus showing the standard magnet in position and an indication of the position of a magnet under test by dot and dash outline;

Fig. 2 is a front elevation of the apparatus;

Fig. 3 is an end elevation of the left end of Fig. 5 is a schematic diagram of the eelctrical connections between the coils and the fiuxmeter;

Fig. 6 is a vertical longitudinal section of the left end of the apparatus taken on the line 6-6 of Fig. 1;

Fig. 7 is a vertical transverse section of the left end of the apparatus, taken on the line 1--1 of Fig. 6;

Fig. 8 is a horizontal section of the apparatus taken on the line 8-8 of Fig. 6 and Fig. 9 is an isometric view of the fiuxmeter movement showing the compensating elements.

Referring to the drawings, it will be seen that the apparatus is provided with a base 20, which may be placed upon any flat surface 2|, both base and surface being of non-magnetic materials. At.

this point, it is well to stress that the entire; apparatus, excepting the magnets, should be:

made of nonmagnetic materials to prevent any spurious electromagnetic efiects. Secured to the; top of the base 20 are two oppositely disposed bearing blocks 22 and 23 having apertures 24 and,

25 formed therein to provide bearings for a hollow tube of non-magnetic metal, which is: journalled in the apertures 24 and 25, and ex tends a substantial distance through and beyond both bearing blocks.

The construction of both ends of the tube 30 being identical, only one end is described and illustrated in detail herein. The inside diameter of the tube 30 is formed slightly greater for a short distance from the end of the tube to receive the enlarged portion of a coil housing 3|, which is generally cylindrical in shape and has one portion of reduced diameter adapted to fit the normal inside diameter of the tube 30. The abrupt changes in diameter of both the tube 30 and the coil housing iorm complementary abutting shoulders 32 and 33, respectively, facilitating the alignment of the coil housing 3| relative to the tube 35. After proper alignment, the coil housing 3| is rigidly secured to the tube 30 by means of the pin 29. A deep transverse groove 35 is provided in the outer end of the coil housing 3| to house a coil wound around a form 4|. The coil form 4| is provided with transverse apertures 42 and 43 at either end to receive insulated bushings 44 and 45, in which are fastened a pair of pins and 5|, which provide mounting support for the coil form, and rigid terminals for the coil. The pins 50 and 5| are fitted in insulating bushing inserts 52 and 53 positioned in apertures 54 and 55 formed in the coil housing 3|. Coil 40 is held in place by a cap secured to the end of the coil housing 3| an insulator 6| being interposed between the coil and the cap to prevent the coil from being shortcircuited through the cap. A secure electrical connection is made between the ends of the coil 40 and the terminal pins 50 and they, in turn. being electrically connected to a pair of leads 52 and 63, which pass through an aperture 54 into the hollow portion of the tube 30 and emerge through an aperture 65 formed at the center of the tube 30. Since there is a coil 66 at the other end of the tube 30, it will be apparent that two pairs of leads, one pair from each coil, will extend through the aperture 65. Each pair of leads should be twisted its entire length up to the coil to counteract the effects of undesired induced currents. A terminal block 61, made from a nonductor, and having an aperture TB coincidental with the aperture 65, is provided on the tube 30 to furnish convenient electrical access to the coils. .the leads of which are connected to terminal posts on the block 61. The coils are electrically connected to each other in series-opposition and in'series with a fiuxmeter H (Fig. 5).

Identical magnet supports and 73, one for a magnet 6 and the other (T3) for a magnet T! to 'be'tested, are adjustably mounted at either end of the base 20 by securing extending arms 74 and of the magnet supports to adjustable angle members Bil-81 and 8233, respectively. Adjustability of the magnet supports is provided by securing the angle members to the base by means of screws 84 and 85 passing through elongated mounting holes 85 and 3? formed in the angle members. Both magnet supports being similar in construction, the detailed description of only oneis herein given. The other of the arm '14, which has a transversely concave top surface, forms the lower central portion of the magnet support 72. Side pieces and '81, having slots 92 and 93, respectively, "formed therein are secured to the arm .14. The edges of the slots 92 and 93 are beveled to receive and provide a seated support for the beveled pole pieces 9!! and 55 of the magnet '15. A block lil'i, having a transversely concave bottom, is fitted between the side pieces 99 and 91 and positioned above the arm 74 so that the concave surfaces of the block 1 ill and of the arm :4 form a substantially cylindrical cavity, into which tube extends, sufiicient clearance being provided so that tube 30 may freely rotate without touching the block it! or arm 14. The magnet support is normally positioned to align the center of the slots 92 and S3 with the center line of the coil 60. The disposition of the coils 4i] and 6% at either end of the tube 30 is such that their longitudinal axes are parallel to each other and cross the longitudinal axis of the tube 30 at right angles thereto. This is done to insure that at all times the angle of rotation around the longitudinal axis of tube 33 of coil with respect to the pole pieces of its associated magnet will be the same as the angle of rotation, about the same axis, of coil 66 with respect to its associated magnet. The magnet supports are of such a height that when the pole pieces of a horn type magnet 15 are fitted into the recesses formed by the slots 12 and 93, the magnet will be suspended free and clear or" the flat surface 2! to prevent any possible misalignment due to the bottom of the magnet touching the surface 2 l. Movable collars H12 and 183, having set screws I84 and 105, encircle the tube 30 and serve to clamp the tube in any predetermined longitudinal position.

To provide means for rotating the tube 30 about its longitudinal axis, an operating lever H0 is securely fastened to the tube 30 near its center and at right angles thereto. Adjustable stops H I and I12 are provided within the arc of rotation of the lever [ID to align the position of the co ls with respect to the pole pieces of the magnets, and to limit the operable rotation of the tube 30 to 180.

Sensitivity of the apparatus is increased by aligning the coils 40 and 56 in such a manner that during the 180 rotation, the greatest change in number of flux linkages will take place. This is accomplished by aligning the coils 40 and .53 so that, at the starting point of the rotation, the circular cross sections of the coils 40 and 66 are at right angles to the lines of force between the pole pieces of the magnets T6 and H, respectively.

The fiuxmeter H is preferably of the type having a minimum of restorative torque. The movement of such a meter is preferably compensated and damped by the counter EMF generated in the moving coil H3 and, in addition, by a small compensating dipole magnet 1 l4 fixed to an extending member H5 depending from the moving coil H3 of the meter, the poles of the dipole being so disposed with-in the field of the meter magnets [H5 that each pole faces a magnetic field of similar polarity emanating from the permanent magnets of the meter. The strength of the field adjacent the dipole may be varied by two adjustable blocks I28 and Hi of magnetic material disposed in the field of the meter magnets. Since the poles of the dipole face similar polar fields, they will be repelled, the repulsion force working against the torque of the meter movement suspension 122. The strength of the field at the dipole should be adjusted so that at any position of the meter movement the reactive torque of the meter suspension (22 will be as nearly as possible equal to the torce of magnetic repulsion at the dipole I M.

In practice, it is convenient to standardize parts of the apparatus to permit comparative magnetic measurements to be made. The coil 65 associated with the support 13 must have a known turnsarea value, which is the product representing the number of turns of the coil multiplied by the circular cross-sectional area of the coil in square centimeters. The desired field strength in gauss is then multiplied by the turns-area value, the product being the number of flux linkages that will be prodced by the coil .66 and a magnet of the desired field strength. The apparatus is standardized bysuspending in the magnet support '12'the magnet 16, the value of which need not be exactly known but, which will, in combination with the coil Ml and after proper adjustment, have a fluix linkage of the same value as the coil 56 would have in combination with a magnet of the desired field strength. After placing the magnet 16 in the support 12. the coil to is rotated through 180 and the change in the flux linkages observed on the .fhmmeter 1!. If the desired change is not indicated, then the support 72 is moved to another position by adjusting the angle pieces and 81. The .coil to is again rotated through and the change in the number of flux linkages again observed. The process is repeated until the desired change in the number of fi-ux linkages is indicated on the flux meter. During the above standardization procedure, there should -be no magnet in the support 13. A magnet 11 to be tested is then suspended in the support 13 and the lever l M swung through a 180 are, thus turning coils 40 and 66 throu h 180. The fluxmeter II will then indicate the difierence between the change in the number of flux linkages produced by the coil 40 and magnet '16 and the change in the number of flux linkages produced by the coil 66 and the magnet '11 under test. The indication on the fluxmeter is not dependent upon the speed of rotation of the coils because the fiuxmeter produces an integration of the changes in flux linkages as the coils move through their respective magnetic fields, making the indication independent of the time required for the flux changes.

Where a number of magnets are to be tested for a particular field strength X, the standardization may be simplified by suspending a standard magnet having X field strength in the magnet support 22 and constructing the coils 49 and 86 to have the same turns-area value. The magnet supports should be adjusted to properly align them with the coils. When a magnet 11 to be tested is placed in the magnet support 13 and the coils rotated through 180, the fluxmeter II will indicate the difference, if any, by which the two magnets vary in field strength. If the magnets are equal, deflection of the fluxmeter will be zero.

What is claimed is:

1. An apparatus for measuring magnetic fields of magnets, which comprises a hollow shaft,

means mounting the shaft rotatably, a standard magnet mounted at one end of the shaft, means for mounting a magnet to be tested :at the other end of the shaft, a search coil keyed to one end of the shaft in the field of the standard magnet, a second search coil keyed to the other end of the shaft in the field of a magnet supported by the mounting means, manually operable means for turning the shaft, means for limiting movement of the shaft to a predetermined angle, a

fiuxmeter for measuring total interlinkages, and

flexible leads extending along the hollow shaft for connecting the coils in series-opposition to the fiuxmeter.

2. An apparatus for measuring magnetic fields of magnets, which comprises a hollow shaft composed of non-magnetic material, means mounting the shaft rotatably, a standard magnet mounted at one end of the shaft, means for mounting a magnet to be tested at the other end of the shaft, a search coil, a plug of non-magnetic material keyed to one end of the shaft and supporting the search coil in the field of the standard magnet, a second search coil, a second plug keyed to the other end of the shaft and supporting the second search coil in the field of a magnet supported by the mounting means, manually operable means for turning the shaft, adjustable stop means for limiting movement of the shaft to a predetermined angle, a fiuxmeter for measuring total interlinkages, and leads having portions extending along the interior of the shaft for connecting the coils in series-opposition to the fluxmeter.

PAUL F. ELARDE.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date Re. 18,555 Sperry Aug. 2, 1932 837,044 Ellis Nov. 27, 1906 I 2,002,680 Scott May 28, 1935 2,202,884 Zuschlag June 4, 1940 2,231,085 Morrison et a1 Feb. 11. 1941 2,231,810 Kamenarovic Feb. 11, 1941 2,291,692 Claud Aug. 4, 1942 2,359,894 Brown et a1 Oct. 10, 1944 2,382,743 Penther et al Aug. 14, 1945 

